1. Field of the Invention
The present invention relates to a technique of controlling operation of a synchronous motor in a sensor-less manner, or more specifically to a technique of controlling operation of a synchronous motor that is driven under a non-loading condition.
2. Description of the Related Art
In a synchronous motor that rotates a rotor through an interaction between a magnetic field occurring when multi-phase alternating currents are flown through windings and a magnetic field produced by permanent magnets, in order to obtain a desired rotational torque, it is required to control the multi-phase alternating currents according to the electrical angle or the electrical position of the rotor. The electrical angle may be detected with a sensor, such as a Hall element. It is, however, desirable to detect the electrical angle and control operation of the synchronous motor in a sensor-less manner, with the view to assuring the reliability of a control apparatus of the synchronous motor.
In the case where the motor is rotated under a non-loading condition, it is not required to make the electric currents flow through the windings.
This case requires neither detection of the electrical angle nor control of the motor. It is accordingly sufficient that switching elements of a driving circuit for making the electric current flow through the windings are all set in the OFF position. Control of the motor resumes when the motor is driven again under a loading condition. This method, however, does not assure accurate detection of the electrical angle on the re-start of the control, which may result in inadequate control of the motor and cause a torque variation. In order to realize a smooth shift of the driving state of the motor from the non-loading condition to the loading condition, it is desirable to detect the electrical angle even in the course of operation of the motor under the non-loading condition.
A proposed method detects the electrical angle according to voltage equations (1) and (2) given below in a sensor-less manner in a salient pole-type synchronous motor especially when the motor is driven at a relatively high speed (hereinafter simply referred to as the high-speed operation): EQU Vd=R.multidot.Id+p(Ld.multidot.Id)-.omega..multidot.Lq.multidot.Iq(1) EQU Vq=R.multidot.Iq+p(Lq.multidot.Iq)-.omega..multidot.Ld.multidot.Id+.omega.. phi. (2)
where V denotes voltages applied to the motor, I electric currents flowing through the windings of the motor, and L inductances of the windings. The subscripts d and q attached to V, I, and L show that the values relate to the d-axis direction and the q-axis direction of the motor. R denotes the coil resistance of the motor, .omega. the rotational angular velocity of the motor, and .phi. the number of flux linkages. Among these arithmetic elements, the coil resistance of the motor R, the inductances L, and the number of flux linkages .phi. are intrinsic to the motor and are thereby referred to as the motor constants. The time derivative operator p is defined as: EQU p(Ld.multidot.Id)=d(Ld.multidot.Id)/dt
The d-axis and the q-axis are described briefly with the drawing of FIG. 4. A permanent magnets-type three-phase synchronous motor is expressed by an equivalent circuit shown in FIG. 4. In this equivalent circuit, the direction that passes through a center of rotation of the motor and is along a magnetic field produced by a permanent magnet is generally referred to as the d-axis. The direction that is perpendicular to the d-axis in a plane of rotation of the rotor is generally referred to as the q-axis. In the equivalent circuit of FIG. 4, the angle of the U phase and the d-axis corresponds to an electrical angle .theta. of the motor.
The proposed method of electrical angle detection using the voltage equations (1) and (2) is briefly described. The voltage equations (1) and (2) are always valid with respect to the d-axis and the q-axis, but the accurate value of the electrical angle is unknown in the case of the sensor-less control of the motor. The motor control apparatus accordingly solves the voltage equations (1) and (2) with an estimated electrical angle (corresponding to .theta.c in FIG. 4). Errors of the arithmetic operations accordingly exist corresponding to an angular error (.DELTA..theta. in FIG. 4) between the estimated electrical angle .theta.c and the true electrical angle .theta.. Successive correction of the estimated electrical angle .theta.c with the errors of the arithmetic operations gives the accurate electrical angle.
The proposed method of electrical angle detection detects the electrical angle in the sensor-less manner with a high accuracy in the case where the permanent magnets-type motor is driven at a relatively high speed. In the synchronous motor in this driving state, however, when the required torque of the motor is substantially equal to zero, that is, when the motor is driven under the non-loading condition, this method has the significantly lowered accuracy of detection of the electrical angle or can not even detect the electrical angle at all.
FIG. 8 shows the results of electrical angle detection when the motor is driven under the non-loading condition. The graph of FIG. 8 shows a comparison between a real electrical angle of the rotating motor actually measured with a sensor and an observed electrical angle detected in a sensor-less manner. As clearly understood from the graph of FIG. 8, the observed angle includes a significant error relative to the real angle.